- The revolution is successful. But survival depends on drastic measures. Your continued existence represents a threat to the well-being of society. Your lives mean slow death to the more valued members of the colony. Therefore, I have no alternative but to sentence you to death. Your execution is so ordered, signed Kodos, Governor of Tarsus IV. — from Kodos' speech to his 4,000 victims.
The Tarsus IV Massacre was one of the most barbarous acts to take place since the founding of the Federation, and served as a reminder that humanity might not be as far removed from its violent past as many would like to think.
In 2246, the colony world of Tarsus IV was the site of a severe crop blight. An exotic fungus destroyed most of the food supply, and then-Governor Kodos was faced with a grim calculus: by rationing the food he could save half the 8000 colonists and visitors until the colony was resupplied, but only if the other half stopped consuming supplies—completely and immediately.
Feeling he had to act quickly and decisively to save the colony (and in the process cutting off other possible options), Kodos and his supporters overthrew the democratic structures of the colony's government and used his personal theories of eugenics to sort the colonists into two halves: one "more necessary" for survival, the other "less necessary". In an act for which history dubbed him "Kodos the Executioner", Kodos ordered the "less necessary" half be put to death.
In a twist of fate, vessels with supplies for the colony arrived ahead of schedule, rendering this ghastly extreme measure completely unnecessary. Kodos' "revolution" was itself overthrown and amid the violence a charred body presumed to be Kodos was found.
Among the survivors were future Starfleet officers James T. Kirk and Kevin Riley, who were children at the time but were among the few who could identify Kodos personally. Eventually it was revealed that another survivor was Kodos himself, exposed shortly before his death at the hands of his own daughter in 2266. (TOS episode & Star Trek 1 novelization: The Conscience of the King; TOS - Enterprise Logs short story: "Though Hell Should Bar the Way"; TOS novel: Final Frontier)
- According to a computer image (seen here) created for the ENT episode: "In a Mirror, Darkly", Hoshi Sato and Takashi Kimura lived on Tarsus IV until they executed in the Tarsus IV Massacre. However, an alternative image not mentioning this was used instead (seen here), rendering these events as apocryphal.